Electric power capacitors are widely used for power factor correction in electric power substations and along transmission and distribution lines. A typical capacitor bank includes three phase banks, one for each electric power phase. Each phase bank includes one or more segments, which each bank includes one or more canisters or “cans” containing a number of internal capacitor packs. Each capacitor pack, in turn, includes a large number of individual electric capacitors. A relatively small capacitor bank connected to a distribution line may only include one capacitor canister per phase, while a large substation may include several segments, which each include multiple capacitor canisters for each phase. For example, a typical substation capacitor bank may include four segments per phase, where each phase includes six capacitor cans for a total of twenty four canisters in the capacitor bank.
Regardless of the number of canisters in a particular capacitor bank, each capacitor canister serves as the separately connected, replaceable unit that is electrically connected, typically by “jumpers,” to an electric power phase (note that a single electric power phase usually includes one conductors or cable, but may include multiple conductors). Each capacitor canister therefore includes two high voltage bushings (also known as insulators) that provide the points of electrical interconnection between the capacitor canister and an electric power phase.
The capacitor packs inside each canister can fail in the ordinary course of operation. Internal fusing typically allows the canister to continue functioning even after an internal capacitor has failed, although the capacitance of the canister will have been reduced. A partially failed capacitor canister connected to only one phase also causes phase imbalance when the capacitor bank is energized. As a general rule, each capacitor canister is typically considered functional when operating with a single internal capacitor failure, and drops out of service requiring replacement when two or more internal capacitors have failed.
In conventional practice, there is no effective way to determine when a capacitor canister has experienced a partial failure, typically involving only one internal capacitor pack, but still remains operational. This is because a partially failed capacitor canister provides no visual indication or easily measured electrical indication of the partial failure. Instead, the conventional practice is to replace a capacitor canister only after it has experienced sufficient internal failures (typically two capacitor failures, which may occur in the same capacitor pack or in two different capacitor packs) to drop out of service, which exposes the system to some period of operation without the capacitor bank in service. At present, there is no economically feasible way for conducting more proactive capacitor canister monitoring, internal fault detection, and replacement.
Present approaches require a technician to take an entire capacitor bank out of service to test each individual canister. Using this approach, capacitor unbalance is detected and canisters are replaced when more than one capacitor pack in a single canister has been determined to have failed based on the measured capacitance of the entire canister. This approach may be too conservative, however, because a capacitor pack with a single failure is typically acceptable, while a single capacitor pack with two or more failed capacitors is prone to explosive failure. It would therefore be desirable to be able to determine whether a multi-capacitor failure has occurred within a single capacitor pack, or whether failed capacitors in a particular canister are distributed among multiple capacitor packs. However, present techniques cannot determine whether two capacitors have failed within a single pack, or whether the failed capacitors are distributed throughout multiple capacitor packs in a particular canister.
In addition, conventional capacitor failure detection techniques identify capacitor failures by measuring capacitive imbalances between the phases. As a result, balanced capacitor failures across the phases tend to re-balance the network and mask the failures, hence producing an apparent good condition leaving a potentially precarious condition undetected. Also, conventional capacitor monitoring techniques require measuring the condition of each capacitor canister while the entire capacitor bank is removed from service, which is time consuming and extends the time that reactive power compensation from the capacitor bank is unavailable. This can be particularly expensive because capacitor failures tend to occur at times of high electricity consumption, when the need for power factor correction by the capacitor bank is high. Testing a large capacitor bank that has experienced a failure typically requires a week or more with entire bank of service for testing, which usually occurs during times of greatest need for the capacitor bank.
There is, therefore, a need for a more effective approach for electric power capacitor monitoring and replacement.